Artistic conception of gravitational waves. Public Domain Image, source: R. Hurt/Caltech-JPL. Yes, gravity can forms waves. Gravitational waves are ripples in
spacetime that travel through the universe. If you think of gravity as a
force acting at a distance, it is difficult to visualize how
gravitational waves could form. However, if you use the more accurate
description of gravity that was developed by Einstein in his general
theory of relativity, these concepts become more logical.
General relativity describes gravity as a warping or curvature of
space and time. All objects warp spacetime. When other objects travel
through this warped spacetime, they end up traveling along curved paths.
These curved paths look like they result from a force being exerted on
the objects, when in reality they result from spacetime itself being
warped. For instance, when you throw a baseball to your friend, it
follows a smooth parabolic trajectory under the influence of gravity.
Isaac Newton’s laws would say that earth’s mass is creating a
gravitational force which acts on the baseball, gradually pulling the
baseball down from straight-line motion. However, the more accurate
description goes like this: The earth warps space and time. The baseball
is actually traveling in a straight line relative to spacetime, but
since spacetime itself is curved, this straight line becomes a curve
when viewed by an external observer. In this way, there is not really
any direct force acting on the baseball. It just looks that way because
of the spacetime warpage. If all of this sounds too strange to be
believed, you should know that Einstein’s general relativity has been
mainstream science for over a hundred years and has been verified by
countless experiments.
In principle, all objects warp spacetime. However, low-mass objects
such as houses and trees warp spacetime to such a small extent that it’s
hard to notice their effects. It takes high-mass objects such as
planets, moons, or stars in order for the gravitational effects to be
noticeable. The more mass an object has, the more it warps spacetime,
and the stronger its gravitational effect on other objects. For
instance, a black hole has such a high amount of mass in such a small
volume that even light cannot escape. Inside the event horizon of a
black hole, spacetime is so strongly warped that all possible paths that
light can take eventually lead deeper into the black hole.
Since spacetime warpage is caused by mass, the warpage travels along
with the mass. For instance, earth warps the surrounding spacetime into
an inward-pinched shape (roughly speaking). As the earth travels around
the sun in its year-long orbit, this pattern of spacetime curvature
travels along with the earth. An observer that is stationary relative to
the sun and is at a point close to earth’s path would see the earth get
closer and then farther away, closer and then farther away, in one-year
cycles. Therefore, this observer would see earth’s pinched spacetime
pattern come closer and then farther away, closer and then farther away,
in one-year cycles. Because the observer himself sits in spacetime and
experiences it, the observer therefore sees his own local spacetime as
being pinched, and then not pinched, pinched and then not pinched, in
one-year cycles. The observer is therefore experiencing an oscillation
of spacetime curvature that is traveling outward from the earth, i.e. a
gravitational wave. This actually happens in the real world. However, in
practice, gravitational waves are so incredibly weak that they have no
significant effect on daily life. The oscillating spacetime warpage of a
passing gravitational wave is far too weak for humans to notice or
feel. Only very sensitive, expensive, modern equipment is able to detect
gravitational waves. In fact, it took a hundred years after Einstein
predicted the existence of gravitational waves for technology to improve
enough to be able to detect them.
This idea of periodically-pinched spacetime is over-simplified. If
you apply the full mathematics of general relativity, you find that an
observer experiencing a passing gravitational wave does not
experience a cycling pattern of spacetime pinching and no pinching.
Rather, the observer experiences a cycling pattern of stretching in the
sideways directions with pinching in the other sideways directions, and
then pinching in the first sideways directions with stretching in the
other sideways directions. For instance, suppose a gravitational wave
from a distant star traveled straight down toward earth’s surface right
where you sit. If the gravitational wave were a thousand trillion times
stronger than it can actually get in the real world, then you would see a
ruler that is aligned with the east-west directions momentarily become
shorter while a ruler that is aligned with the north-south directions
momentarily become longer. And then a moment later, the east-west ruler
would become longer while the north-south ruler would be shorter. Each
ruler would continue to get periodically longer and shorter until the
gravitational wave has passed. There is nothing wrong with the rulers.
Spacetime itself is warping and everything in spacetime experiences the
warping.
Although this effect is very weak, it actually happens. A
gravitational wave detector is effectively just a very long ruler with
the ability to measure the length of the ruler very accurately. For
instance, each arm of a LIGO detector is 2.5 miles long and uses lasers
to accurately measure lengths. Even with large, modern, expensive
detectors, gravitational waves are so weak that only the largest waves
can currently be detected. The current detectors cannot pick up the
gravitational waves generated by planets orbiting stars or moons
orbiting planets. The largest gravitational waves are generated when two
black holes orbit each other rapidly immediately before falling
together and merging. Large waves are also generated when two neutron
stars orbit each other, or when a black hole and a neutron star orbit
each other, immediately before merging. These are the only types of
gravitational waves that have been detected so far.
In general, a gravitational wave is created any time a mass
accelerates. Traveling along a circular path is only one type of
acceleration. If an object with mass speeds up along a straight path,
this is also a type of acceleration, and therefore it should create
gravitational waves. Similarly, an object with mass slowing down along a
straight path should also create gravitational waves. However, on the
astronomical scale, an object traveling steadily along a circular orbit
is far more common than an object violently slowing down or speeding up.
Another point to keep in mind is that the gravitational waves created
by the earth in its yearly orbit are not only extremely weak, they also
have a period of one year. This means that a gravitational wave
detector on another planet would have to watch for several years in
order to pick up the oscillatory shape of the gravitational waves
generated by earth’s orbital motion. In contrast, immediately before two
black holes merge, they orbit each other so rapidly that it only takes a
fraction of a second for each to complete an orbit. This is another
factor that makes these types of gravitational waves easier to detect.Credit:wtamu.edu
